This invention relates to an improved method of forming a film on a substrate surface by chemical vapor deposition (CVD) using at least two kinds of reactant gases, such as an organosilicon compound gas and oxygen gas containing ozone in the case of depositing a silicon oxide film.
In the fabrication of semiconductor devices, CVD is widely employed for depositing a silicon oxide film on a substrate surface. With a view to depositing silicon oxide films of good properties at fairly high deposition rates at relatively low temperatures, recently attention has been devoted to the use of tetraethylorthosilicate (abbreviated to TEOS) Si(OC.sub.2 H.sub.5).sub.4 or an alternative organosilicon compound and ozone as the principal reactants to carry out CVD.
The deposition of a silicon oxide film by reaction between TEOS gas and ozone, as an oxygen gas containing ozone, can be accomplished by atmospheric pressure CVD. Known atmospheric pressure CVD methods using TEOS and ozone are classified broadly into two categories, viz. premix methods shown, e.g., in IEDM-89 (1989), pp. 669-672 and Journal of Electrochemical Society, Vol. 138, No. 10(1991), pp. 3019-3024 and post-mix methods shown, e.g., in Denkikagaku, Vol. 56, No. 7(1988), pp. 527-532 and Journal of Electronic Materials, Vol. 19, No. 1 (1990), pp. 45-49.
In the premix method, TEOS gas and an ozone-oxygen gas are mixed in a gas mixer which is provided outside the CVD reaction chamber and kept heated, and the resultant gas mixture is fed into the reaction chamber through a suitably heated pipe. By this method TEOS makes contact with ozone for a relatively long period of time, and TEOS begins to react with ozone within the gas mixer or the piping connecting the mixer to the reaction chamber. Therefore, oxidation of TEOS proceeds almost to the full extent, and this is effective for depositing a silicon oxide film with very good in film properties and low in moisture content. Furthermore, when the substrate surface has steps such as aluminum wiring lines, the film deposited by the premix method is fairly good in conformality to the steps. This is probably attributed to the formation of intermediate reaction products in the gas mixture of TEOS and ozone before the gas mixture approaches the substrate surface.
However, the premix method has disadvantages which are also attributed to the early commencement of reaction between TEOS and ozone. That is, as the CVD operation is repeated solid oxides formed by the reaction deposit and accumulate in the piping through which the TEOS and ozone mixture is fed and gas injection nozzles in the reaction chamber. Consequently the piping and the nozzles deteriorate, and changes occur in the rate of film deposition on the substrate surface and the properties of the deposited film. Besides, adhesion of contaminating particles to the deposited film increases. These problems become very serious when the ozone concentration in the ozone-oxygen gas is relatively high.
In the post-mix methods TEOS gas and an ozone-oxygen gas are separately introduced into the reaction chamber by using a group of nozzles for injecting TEOS gas and another group of nozzles for injecting the ozone-oxygen gas such that the two gases mix with each other just in the vicinity of the substrate surface. Naturally this method is free from the above described disadvantages of the premix method.
However, by the post-mix method it is difficult to deposit a silicon oxide film that has very good film properties and conformality to underlying steps, because TEOS is allowed to react with ozone only for a very short period of time. To improve the film properties it is necessary to greatly increase the ozone concentration, but various problems arise when a very high ozone concentration is employed.